Recently, several methods have been known as a lock-up clutch control device for an automatic transmission of a vehicle. For example, as disclosed in JP07-239025A2, to carry out quick lock-up operation with restraining shock, a lock-up clutch control device judges whether a dead zone of controlling in loss-stroke zone of the lock-up clutch (friction material) or a controllable zone that the lock-up clutch is in contacting state by means of an estimation system of a clutch system. In other words, this estimation system estimates contacting torque (divided torque) of the lock-up clutch, then judges contact timing of the lock-up clutch (namely an end point of a loss stroke zone) on the basis of estimated contacting torque.
In the disclosed lock-up clutch control device, to estimate contacting torque of the lock-up clutch, the disclosed lock-up clutch control device refers engine torque, friction coefficient μ of the friction material, performance (specification) of a torque converter and so on. Therefore, when variation of the engine torque or change of the friction coefficient by means of aging or temperature change exists, it may be difficult to judge the end point of the loss-stroke zone (contact timing of the lock-up clutch) exactly.
Therefore, a need exists for a lock-up clutch control device to estimate adequate contact timing of a friction material.
On the other hand, another example of a lock-up clutch control device has been disclosed (JP2827691B2). This lock-up clutch control device executes learning control of output wave form (precharge control wave form) in precharge controlling of hydraulic pressure to carry out quick lock-up operation with restraining shock.
In the disclosed lock-up clutch control device, the precharge wave form is defined as a combination of a first period wave form in which a pressure in the order of maximum pressure is kept as a control value of lock-up hydraulic pressure for precharge control and a second period wave form in which ramp control pressure is set after the first period with a predetermined time inclination, then lock-up operation pressure is precharged.
Therefore, since the second period wave form includes ramp inclination and shock at contacting is different between a case that the end point of the loss-stroke zone is provided at initial stage of ramp control and a case that the end point of the loss-stroke zone is provided after passing the predetermined time of ramp control. For example, if ramp initial pressure (namely, the second period wave form) is learned based on the shock at contacting, it may be difficult to define degree of compensation.
Therefore, a need exists for a lock-up clutch control device to execute adequate earning of a precharge control wave form in precharge control against loss-stroke.